CE398 - THEORY OF STRUCTURES INFLUENCE LINE FOR TRUSSES Prepared by Engr. Jason Lim TRUSSES A Truss is a structure composed of slender members joined together at their end points. The member commonly used in construction consist of wooden struts or metal bars. A truss is a structure that consist of members assembled into connected triangles so that overall assembly behaves a single object. Trusses are most commonly used in bridges, roofs and towers TRUSS LOADING The loading on the bridge deck is transmitted to stringers, which in turn transmit the loading to floor beams and then to the joints along the bottom cord of the truss. Since the truss members are affected only by the joint loading, we can therefore obtain the ordinate values of the influence line for a member by loading each joint along the deck with a unit load and then use the method of joints or the method of sections to calculate the force in the member. SIGN CONVENTION Tensile (+) = Positive Compressive (-) = Negative SAMPLE PROBLEM 1 Draw the influence line for the force in member GB of the bridge truss SAMPLE PROBLEM 1 Draw the influence line for the force in member GB of the bridge truss SAMPLE PROBLEM 2 Draw the influence line for the force in member CG of the bridge truss SAMPLE PROBLEM 2 Draw the influence line for the force in member CG of the bridge truss SAMPLE PROBLEM 3 In order to determine the maximum force in each member of the Warren truss, shown in the photo, we must first draw the influence lines for each of its members. If we consider a similar truss as shown in the next figure, determine the largest force that can be developed in member BC due to a moving force of 25 k and a moving distributed load of 0.6 k/ft. The loading is applied at the top cord. SAMPLE PROBLEM 3 SAMPLE PROBLEM 3 FBC(max) = 49.0 k SAMPLE PROBLEM 4 FBC(max) = 49.0 k SAMPLE PROBLEM 5 Determine the maximum compressive force developed in member BG of the truss in figure due to the wheel loads of the car and trailer. Assume the loads are applied directly to the truss and move only to the right. SAMPLE PROBLEM 5 SAMPLE PROBLEM 5 SAMPLE PROBLEM 5
